CAST IRON INDEFINITE CHILL ROLL PRODUCED BY THE ADDITION OF NIOBIUM

Description

BACKGROUND OF THE INVENTION

Field of the Invention

[0001] The invention relates to a process for producing a chill roll having surface properties that are highly desirable for use in the hot rolling of steel. More particularly, the invention relates to the discovery that the introduction of niobium into a chilled-iron roll casting composition produces surface hardness values not previously attainable without interfering with the balance between carbide formation and free graphite dispersion that is necessary in such casting compositions.

Background of the Invention

[0002] In the continuous hot rolling of steel strip, a continuously moving steel workpiece (the strip) is passed through a rolling mill which commonly consists of several stands of rolls arranged in a straight line (in tandem). The strip cools as it passes through the rolling mill, such that each succeeding stand is at a lower temperature than its predecessor stand.
Typically, when the strip reaches the rolls of the last few mill stands there is a tendency of the strip to weld or fuse to the rolls through which it passes because of the lower temperature of the roll. The results of such welding can be a catastrophic demolition of the rolling mill stands and surrounding structures, not to mention the grave threat to workers in the area.

[0003] It is evident, therefore, that the selection of the proper grade of roll to be used in the latter stands of tandem style rolling mills is important. The problem of roll selection is complicated by the fact that mill conditions vary widely, but in general the finishing rolls on a tandem hot mill should have an outer skin which is dense and hard, and yet provide sufficiently low friction in the areas that contact the workpiece.

[0004] Since the early days of steelmaking, rolling mill rolls have been cast in a manner to ensure that the liquid iron on the outer surface of the roll is cooled to produce the desired structure and properties. One technique for attaining this rapid cooling is to insert metal rings or segments, called "chills", in the mold, close to the surface to be contacted by the molten iron. The production of the chill roll shells typically involves a two step process, in which an outer shell in formed that possesses the aforementioned qualities necessary for use in a rolling mill followed by the formation of an inner core composed of a material that provides additional strength to the chill roll, such as cast iron. The outer shell is formed by either a static or spin pour, as is well known in the industry, an example of which is U. S. Patent 5,355,932 issued to Nawata et al.

[0005] Most early chill rolls were cast using ordinary low silicon iron alloyed with nickel and chromium and chilled at a very high rate to suppress the formation of graphite, which was thought to be detrimental to the roll due to the softness imparted to the alloy by the graphite. The chilled outer surface is very hard and, when fractured, has a white fracture face for a distance beneath the surface (known as the chill zone), signifying that the formation of free graphite in that area had been suppressed by the rapid cooling. The white iron zone sometimes is referred to as "white cast iron", as contrasted with iron containing graphite that has a grey fracture face, known as "grey iron".

[0006] In the 1930s, it was discovered that the introduction of finely dispersed graphite into the white iron zone substantially reduced roll breakage despite providing for a softer outer shell. The region of the finely dispersed graphite in the alloy is termed "mottled." The presence of graphite in the outer shell greatly improves the ability of the roll to withstand the thermal shocks associated with hot rolling steel strip, reduces the friction between the roll and the strip thereby lowering the applied stress on the strip, and greatly reduces the potential for fusing of the strip to the roll. As a result, white cast iron chill rolls were largely superseded by a roll characterized by finely dispersed graphite near the outer surface of the roll and the lack of a definite chill zone. Such a roll has become known as an "indefinite chill" roll (or a "grain" roll).

[0007] While indefinite chill rolls significantly improve the durability of the roll over white cast chill rolls, the presence of graphite provides for a softer roll having a lower wear resistance and a shorter usable life between regrinds than the more highly alloyed rolls in the same finishing stands. Considerable efforts have been made worldwide to develop rolls which do not weld to the steel strip being rolled and have a better resistance to abrasion than the indefinite chill rolls. A primary focus of the efforts is on the use of metallic carbides to increase the hardness and abrasion resistance of an iron alloy as is known in the art; however, increasing the amount of carbides generally produces a commensurate reduction in the amount of graphite in the alloy. Numerous attempts have been made to develop alloys containing potent combinations of strong carbide forming elements, such as are used in tool steels, to replace the indefinite chill roll compositions. However, these high carbide, low graphite alloy rolls have also proven to be unsuitable for chill roll applications, because of the tendency to weld to the material being rolled and to initiate pressure cracks, much like the white cast iron chill rolls. For lack of a superior alternative, indefinite chill rolls have been retained in the late finishing stands of many of the modern high speed hot strip mills and the use of potent carbide forming elements has been limited to relatively small additions, usually of molybdenum, to indefinite chill roll compositions to alter the matrix structure or extremely small additions of magnesium to control the form of the graphite.

[0008] EP-A-525 932 discloses a cast iron for a chill roll shell, which has a composition according to the invention and optionally comprises 0.1-2.0 % niobium, the content of free graphite being, however, not mentioned.

[0009] An essential feature of indefinite chill rolls is the critical balance between alloying elements such as carbon, nickel and silicon which promote the formation of graphite and carbide forming elements such as chromium. The formation of an alloy containing the proper balance of graphite and carbides requires extremely careful selection of melting stock, closely controlled melting conditions, rigid control of composition and inoculation techniques to obtain the required type and distribution of graphite. This relationship has inhibited the use of more potent carbide forming elements, which greatly skew the graphite/carbide balance in favor of carbide formation and render the alloy unsuitable for use in indefinite chill roll applications. Thus, for overfour decades the use of potent carbide forming alloys has been inhibited by the overwhelming need to maintain free graphite in the chilled structure of this type of roll.

[0010] Many other applications require the characteristics embodied in indefinite chill rolls, such as in plate mills, temper mills, narrow strip, backup rolls, bar mills for rolling flats, Steckel mills and a variety of cold temper mills. In all of these applications the present advantages of this type of roll would be greatly enhanced by a significant improvement in its resistance to abrasion.

Summary of the Invention

[0011] The invention is defined in any of claims 1, 7 and 9, optional features thereof being set out in the dependent claims. An indefinite chill roll alloy composition is disclosed containing at least 3.3 wt% carbon (all percentages herein being by weight of the alloy unless otherwise stated) of the alloy and the carbon is present as free graphite in an amount ranging from 2-7%, preferably 3-6%, of the total carbon. The composition further includes niobium which ranges from 1.0 - 6.0% and is present essentially as discrete niobium carbide particles in the alloy. A method for producing a chill roll shell formed from the alloy according to the invention includes the steps of (i) providing a molten iron alloy composition, (ii) adjusting the composition by adding niobium in an amount sufficientto produce a molten batch (iii) containing 1.0 to 6.0% niobium based on the total weight of said molten batch, providing a stoichiometric amount of excess carbon to form niobium carbide and free graphite on cooling, and (iv) casting the molten batch to form the chill roll shell. The method of the present invention may be useful to form indefinite chill roll containing significant quantities of carbides from other element that form carbides having low carbide solubilities near the eutectic point of the iron alloy, while maintaining sufficient free graphite in the alloy to produce an alloy have the properties required for chill roll applications.

[0012] The niobium indefinite chill roll composition greatly enhances the abrasion resistance of the indefinite chill type of roll without reducing its resistance to welding to the strip or its resistance to initiation of cracks under shock loading, by maintaining a balance between free graphite and carbides in the chilled zone during eutectic solidification.

[0013] In accordance with the present invention, the use of niobium allows the addition of a relatively large amount of a strong carbide forming element to a roll alloy which will retain its essential partially graphitized chilled structure. A consideration of the partitioning coefficients of other alloys which form carbides at high temperatures suggest that tantalum might also be suitable. Contrariwise, vanadium, tungsten, titanium, molybdenum, and chromium could be expected to dramatically upset the graphite-carbide balance during eutectic solidification and have not be suitable for chill roll applications. Thus, the present invention provides an indefinite chill roll composition that overcomes the problems associated with the prior art. These and other details, objects, and advantages of the invention will become apparent as the following detailed description of the present preferred embodiment thereof proceeds.

Detailed Description of Preferred Embodiments

[0014] As used herein, the term "indefinite chill roll" composition shall mean an iron-based alloy intended for use in casting the shell of a rolling mill roll and generally having the composition:

TABLE 1

KNOWN INDEFINITE CHILL ROLL COMPOSITIONS AND ROLLS FORMED THEREFROM
Constituent	Weight Percent (wt%) (based on the total weight of the alloy)
Carbon	2.5 - 3.6
Nickel	4.2 - 4.6
Molybdenum	0.3 - 0.5
Chromium	1.5 - 2.0
Silicon	0.7 - 1.2
Manganese	0.7 - 1.0
Phosphorus	<0.07
Sulfur	<0.08
Iron and Impurities	Balance

[0015] Alloys of this composition are well known in the art and will produce a proper balance or equilibrium between carbide formers and free graphite formers at the eutectic solidification temperature which is in the range of 1130°C to 1150°C. The resulting alloy contains approximately 30-38% of the total carbon in the form of carbides, approximately 2-7% of the total carbon in the form of graphite and the remaining carbon is alloyed with the iron in the matrix of the alloy. Alloys having graphite present in quantities greater than 7% of the total carbon are generally too soft to be employed as the outer shell of the rolling mill roll, while alloys containing less than 2% free graphite are not suitable to be deployed as a chill roll outer shell because they are not sufficiently resistant to thermal shock and do not have sufficient graphite to reliably prevent welding of the workpiece to the roll. The alloy produced from the indefinite chill roll compositions have a hardness value ranging from approximately 70 to 82 Shore C over the range of carbon used in the alloy.

[0016] Ni is added to the indefinite chill roll composition to promote the formation offree graphite in the alloy; however, an excess of Ni will tend to destabilize the structure of the alloy. Mo is important in the formation of the matrix structure and for controlling the size of the carbides formed in the cast, but Mo is also a potent carbide forming element, therefore Mo must be controlled to minimize excess amounts of Mo that will shift the graphite/carbide equilibrium almost entirely in favor of carbide formation. Cr is also a carbide forming element, but will not skew the graphite/carbide balance as strongly in favor of carbide formation as potent carbide forming elements, such as V, if a balance is maintained with graphite promoting elements. Si and Mn are deoxidation agents that contribute to the formation of graphite and to maintaining the character of the cast, but will have an adverse affect on the crack resistance of the alloy, if present in higher amounts. P and S are generally present as contaminants in the alloy and should be minimized to a practical extent in the alloy, such as to less than 0.07% and 0.08%, respectively. The skilled practitioner will appreciate that minor changes to the elemental ranges and also substitution of comparably active elements can be made to the indefinite chill roll composition, while maintaining the desired properties characteristic of indefinite chill compositions containing 2-7% of the total carbon as free graphite in the alloy.

[0017] While indefinite chill rolls can be produced within the above ranges, the composition and resulting properties of the chill roll can be more easily controlled and are more desirable if the compositional ranges are limited to those shown in Table 2, resulting in an alloy containing 3-6% of the total carbon as free graphite.

TABLE 2

PREFERRED INDEFINITE CHILL ROLL COMPOSITIONS AND ROLLS FORMED THEREFROM
Constituent	Weight Percent
Carbon	3.3 - 3.4
Nickel	4.3 - 4.6
Molybdenum	0.3 - 0.5
Chromium	1.6 - 1.8
Silicon	0.7 - 0.9
Manganese	0.7 - 0.9
Phosphorus	<0.07
Sulfur	<0.08
Iron and Impurities	Balance

THE ADDITION OF NIOBIUM

[0018] In the temperature range of the eutectic point of the molten indefinite chill roll compositions, niobium carbide has a very low solubility. The applicants have discovered that by adding niobium to the molten alloy and by cooling the molten alloy above the eutectic solidification temperature at a rate of not more than about 1°C/sec nearly all of the niobium will precipitate in the form of discrete niobium carbide particles and the solid niobium carbide does not affect eitherthe chemistry of the remaining molten alloy or the formation of other precipitates upon the cooling of the remaining molten alloy to the eutectic temperature. Further, because solid niobium carbide particles are extremely hard (Vickers hardness above 2000), the presence of the carbides in the alloy substantially increases the abrasion resistance of the alloy. Niobium carbide is particularly effective in enhancing the hardness and abrasion resistance of the alloy because the particles have a density of approximately 7.8 g/cc which is very close to that of iron; therefore, the carbide particles will evenly distribute throughout the alloy matrix and will not either float or settle when the outer shell is formed either by static or spin pouring. The uniform distribution of the niobium carbide within the shell is especially important because the outer shell can withstand a number of surface regrinds to smooth the surface without a degradation in the physical characteristics of the shell. Niobium can be added to the alloy over a broad range of indefinite chill roll compositions as shown below:

TABLE 3

NIOBIUM CONTAINING INDEFINITE CHILL ROLL COMPOSITIONS AND ROLLS FORMED THEREFROM
Constituent	Weight Percent
Carbon	3.3 - 4.0
Niobium	1.0 - 6.0
Nickel	4.2 - 4.6
Molybdenum	0.3 - 0.5
Chromium	1.5 - 2.0
Silicon	0.7-1.2
Manganese	0.7 - 1.0
Phosphorus	<0.07
Sulfur	<0.08
Iron and Impurities	Balance

[0019] Another consequence of this discovery is that the once delicate equilibrium between graphite and carbides can now be manipulated using niobium to achieve a wide range of graphite to carbide ratios. Generally, manipulation of the graphite to carbide ratio can presumably be performed using any other carbide forming elements that have low carbide solubilities in molten indefinite chill roll alloy composition above the eutectic temperature. For example, elements having properties similarto niobium, such as tantalum, may also form carbides that have low solubility in molten indefinite chill roll compositions and could presumably function in a manner similar to niobium.

Preparation of the Alloy

[0020] Niobium carbide indefinite chill roll compositions can be prepared in a manner similar to methods typically used to prepare indefinite chill roll compositions. The niobium can be added to the alloy before or after the alloy is melted and in any form, such as niobium metal, ferro-niobium or niobium carbide, that will not shift the overall composition of the alloy to outside the prescribed ranges. The formation of niobium carbide requires that a stoichiometric amount of excess carbon be provided to produce the niobium carbide, while maintaining the desired carbon levels in the indefinite chill roll composition. Preferably, niobium and carbon are added in the form of niobium carbide that will be dissolved in the molten alloy and then precipitate upon cooling of-the molten alloy. Ferro-niobium can also be used; however, excess carbon must also be added and the compositional ranges of the other alloying elements must take into account the addition of iron with the niobium. Niobium metal is not as desirable as either niobium carbide or ferroniobium, because of the high melting temperature of the metal.

[0021] The preparation of the alloy requires heating a metal charge having an overall compositional range required for indefinite chill rolls, stated above, and including an amount of niobium and carbon to form the desired quantity of niobium carbide to approximately 1515°-1540°C in an induction furnace for approximately 30-60 minutes or until an analysis of the molten metal indicates that the molten alloy is within the specifications. At which time, the molten alloy is cooled at a rate of approximately 1°C/sec until essentially all of the niobium carbide has precipitated from the molten alloy and the cooling is continued at a rate of approximately 0.25°C/sec until the eutectic point is reached and solidification of the remaining alloy occurs. In the preparation of the niobium containing alloys, a preferred range of alloy compositions shown in Table 4 were found to be more easily produced according to the aforementioned procedure and result in an alloy containing 3-6% of the total carbon as free graphite.

TABLE 4

PREFERRED NIOBIUM CONTAINING INDEFINITE CHILL ROLL COMPOSITIONS AND ROLLS FORMED THEREFROM
Constituent	Weight Percent
Carbon	3.3 - 3.7
Niobium	1.0 - 3.0
Nickel	4.3 - 4.6
Molybdenum	0.3 - 0.5
Chromium	1.6 - 1.8
Silicon	0.7 - 0.9
Manganese	0.7 - 0.9
Phosphorus	<0.07
Sulfur	<0.08
Iron and Impurities	Balance

Examples

[0022] A cast iron alloy was prepared in the aforementioned manner having the following compositional range:

Carbon	3.3 - 3.4%
Nickel	4.5 - 4.6%
Chromium	1.9 - 2.0%
Molybdenum	0.4 - 0.5%
Silicon	0.7 - 0.8%
Manganese	0.9 - 1.0%
Phosphorus	0.03 - 0.04%
Sulfur	0.05 - 0.06%

[0023] The resulting alloy had a hardness of 80 (Shore C). Using this alloy as a baseline indefinite chill roll composition, a number of niobium carbide alloy were cast by adding increasing amounts of ferro-niobium to the alloy without compensating forthe carbon consumed in the niobium carbide precipitation or the additional iron introduced. The alloys were tested for hardness, the results of which are shown in Table 5 in comparison with the baseline alloy (alloy 0). Also included in the table is the calculated amount of carbon remaining in the eutectic solid taking into account the carbon consumed by the niobium and the addition of iron with niobium, assuming that all of the niobium precipitated as niobium carbide and using the average of the observed ranges for each element.

TABLE 5

HARDNESS OF ALLOY CAST IRON AS A FUNCTION OF NIOBIUM CONTENT
Alloy Sample Number	% Niobium	Hardness (Shore C)	% Carbon Remaining in Alloy Matrix
*0	0.0	80	3.35
*1	0.55	83	3.27
2	1.47	83	3.13
3	3.73	81	2.79
4	4.21	79	2.71
5	5.34	78	2.53
6	5.82	76	2.45

*outside of Invention

[0024] As shown in Table 5, the addition of even a small quantity (0.55%) results in significant improvement in the hardness. However, when the amount of niobium is increased without compensating for the consumption of carbon, the hardness of the material substantially decreases as with samples 4, 5, and 6. The significant effect of the decrease in the carbon content of the remaining alloy is indicative of the delicate balance sought to be achieved in the indefinite chill roll compositions. The addition of nearly 6% niobium results in an alloy having a hardness of only 76 Shore C, which is less than that of the baseline alloy, but which compares favorably to an alloy containing only 2.45% carbon in the matrix-without niobium carbide present in the alloy. In general, the addition of niobium increases the hardness of the alloy by approximately 3 Shore C, which more importantly amounts to a significant increase in the abrasion resistance of the indefinite chill roll composition, while maintaining the necessary amount of free graphite in the alloy to function as a chill roll. The data in table 5 shows a maximum hardness is achieved when the niobium content ranges from 0.55 to 1.47 wt% and the carbon content ranges from 3.27 to 3.13 wt% of the total alloy. Additional testing indicates that the niobium content preferably ranges from 1.0 to 3 wt%, most preferably about 1.5 wt%, when the carbon content ranges from 3.3 - 3.45 wt%.

[0025] In addition, several chill rolls were prepared from the above alloys having dimensions approximately 76.5 cm (30.5 inches) in diameter and 177.8 cm (70 inches) long. One chill roll composed of the alloy containing niobium was placed in the last stand of a rolling mill and tested for comparison with an indefinite chill roll of the prior art, the results of which are shown in Table 6 below:

TABLE 6

INDEFINITE CHILL ROLL WEAR TESTING
Roll Type	Number of Times in the Mill	Metric Tons of steel rolled per Millimeter of wear due to rolling and regrinding	Millimeters of wear per Time in the mill
Niobium containing Alloy	108	2738	0.71
Prior Art	960	1889	1.05

[0026] As shown in Table 6, the niobium carbide indefinite chill rolls greatly increase the life expectancy by about 45% over existing chill rolls based on the metric tons of steel rolled per millimeter of wear due to rolling of the steel and regrinding of the roll between times or trips in the mill. In addition to increasing the length of time between shutting down the mill in order to regrind the chill roll, the niobium carbide chill roll results in a more consistent surface finish to the strip between regrinding because of the lower amount of wear in the surface of the roll.

[0027] Those of ordinary skill in the art will appreciate that the present invention provides significant advantages over the prior art. In particular, the subject invention provides an indefinite chill roll that has increased abrasion resistance, thereby allowing for longer periods of operation before regrinding of the roll is necessary. The invention also provides for the production of a smooth workpiece because of the lower tendency for abrasions to form in the surface of the roll. The subject invention also increases the hardness of the indefinite chill roll, which further provides for a smoother workpiece.

Claims

1. An alloy composition suitable for a chill roll or chill roll shell, said alloy composition consisting of:

1.0 to 6.0 weight % niobium;

at least 3.3 weight % carbon;

and optionally

4.2 to 4.8 weight % nickel;

0.3 to 0.5 weight % molybdenum;

1.5 to 2.0 weight % chromium;

0.7 to 1.2 weight % silicon;

0.7 to 1.0 weight % manganese; and

iron and impurities,

wherein 2 to 7% of said carbon in said alloy composition is present as free graphite.

2. The alloy composition of claim 1, comprising:

3.3 to 4.0 weight % carbon;

1.0 to 6.0 weight % niobium;

4.2 to 4.6 weight % nickel;

0.3 to 0.5 weight % molybdenum;

1.5 to 2.0 weight % chromium;

0.7 to 1.2 weight °/9 silicon;

0.7 to 1.0 weight % manganese; and

iron and impurities.

3. The alloy composition of claims 1 or 2, comprising 1.0 to 3.0 weight % niobium.

4. The alloy composition of any of claims 1-3, comprising about 1.5 weight % niobium.

5. The alloy composition of any of claims 1-4, comprising 3.3 to 4.0 weight % carbon.

6. The alloy composition of any of claims 1-5, comprising 3.3 to 3.45 weight % carbon.

7. A method for producing a chill roll shell formed of alloy cast iron, as defined in any of claim 1-6, the method comprising:

providing a molten iron alloy composition;

adjusting said iron alloy composition by adding at least one of niobium and a niobium-containing compound to provide a molten batch containing 1.0 to 6.0 weight % niobium, and providing a stoichiometric amount of excess carbon in said molten batch to form niobium carbide and free graphite on cooling, said molten batch Including at least 3.3 weight % carbon; and

casting said molten batch to from said chill roll shell containing precipitated niobium carbide and wherein 2 to 7% of said carbon is precipitated as free graphite.

8. The method of claim 7, wherein casting said molten batch comprises cooling said molten batch at not greater than about 1°C/sec. until substantially all niobium carbide precipitates.

9. A method of forming an iron alloy composition as defined in any of claims 1-6 suitable for a roll of chill roll shell, the method comprising:

(i) providing an indefinite chill roll composition having a eutectic solidification point at which a desired graphite content forms;

(ii) adjusting said composition by adding (a) at least one of niobium and a niobium-containing compound and (b) a stoichiometric amount of excess carbon to form niobium carbide, so that the adjusted composition comprises 1.0 to 6.0 weight % niobium and at least 3.3 weight % carbon;

(iii) producing a molten batch from said adjusted composition at a temperature above the eutectic solidification point;

(iv) lowering the temperature of said molten batch to precipitate said niobiurr carbide above the eutectic solidification point; and

(v) further cooling said molten batch to form an amount of graphite which is 2 to 7% of the total carbon in said molten batch.

10. The method of claim 9, wherein lowering the temperature of said molten batch comprises lowering the temperature at not greater than 1°C/sec. until substantially all of said carbide precipitates.

Ansprüche

1. Für eine Hartgusswalze oder ein Hartgusswalzengehäuse geeignete Legierung, bestehend aus:

1,0 bis 6,0 Gew.-% Niob;

mindestens 3,3 Gew.-% Kohlenstoff;

und gegebenenfalls

4,2 bis 4,6 Gew.-% Nickel;

0,3 bis 0,5 Gew.-% Molybdän;

1,5 bis 2,0 Gew.-% Chrom;

0,7 bis 1,2 Gew.-% Silicium;

0,7 bis 1,0 Gew.-% Mangan; und

Eisen und Verunreinigungen,

wobei 2 bis 7% des in der Legierung vorhandenen Kohlenstoffs als freier Graphit vorliegen.

2. Legierung nach Anspruch 1, dadurch gekennzeichnet, dass sie folgendes umfasst:

3,3 bis 4,0 Gew.-% Kohlenstoff;

1,0 bis 6,0 Gew.-% Niob;

4,2 bis 4,6 Gew.-% Nickel;

0,3 bis 0,5 Gew.-% Molybdän;

1,5 bis 2,0 Gew.-% Chrom;

0,7 bis 1,2 Gew.-% Silicium;

0,7 bis 1,0 Gew.-% Mangan; und

Eisen und Verunreinigungen.

3. Legierung nach Anspruch 1 oder 2, dadurch gekennzeichnet, dass sie 1,0 bis 3,0 Gew.-% Niob enthält.

4. Legierung nach einem der Ansprüche 1-3, dadurch gekennzeichnet, dass sie etwa 1,5 Gew.-% Niob enthält.

5. Legierung nach einem der Ansprüche 1-4, dadurch gekennzeichnet, dass sie 3,3 bis 4,0 Gew.-% Kohlenstoff enthält.

6. Legierung nach einem der Ansprüche 1-5, dadurch gekennzeichnet, dass sie 3,3 bis 3,45 Gew.-% Kohlenstoff enthält.

7. Verfahren zur Herstellung eines Hartgusswalzengehäuses, gebildet aus einer Gusseisenlegierung nach einem der Ansprüche 1-6, umfassend:

Bereitstellung einer geschmolzenen Eisenlegierung;

Einstellung der Eisenlegierung durch Zugabe von mindestens Niob und/oder einer Niob enthaltenden Verbindung, um eine geschmolzene Charge, enthaltend 1,0 bis 6,0 Gew.-% Niob, zur Verfügung zu stellen, und Vorsehen einer stöchiometrischen Menge von überschüssiger Kohle in der geschmolzenen Charge, um Niobcarbid und freien Graphit beim Abkühlen zu bilden, wobei die geschmolzene Charge mindestens 3,3 Gew.-% Kohlenstoff enthält; und

Gießen der geschmolzenen Charge, um das Hartgusswalzengehäuse, enthaltend ausgefälltes Niobcarbid, zu bilden, und wobei 2 bis 7% des Kohlenstoffs als freier Graphit ausgefällt sind.

8. Verfahren nach Anspruch 7, dadurch gekennzeichnet, dass das Gießen der geschmolzenen Charge das Abkühlen der geschmolzenen Charge mit nicht mehr als etwa 1°C/s erfolgt, bis im Wesentlichen das gesamte Niobcarbid ausfällt.

9. Verfahren zur Bildung einer für eine Hartgusswalze oder ein Hartgusswalzengehäuse geeigneten Eisenlegierung, die wie in einem der Ansprüche 1 bis 6 definiert ist, umfassend:

(i) Bereitstellung einer unbestimmten Hartgusswalzenzusammensetzung, die einen eutektischen Verfestigungspunkt hat, bei dem sich ein gewünschter Graphitgehalt bildet;

(ii) Einstellung der Zusammensetzung durch Zugabe (a) von mindestens Niob und/oder einer Niob enthaltenden Verbindung und (b) einer stöchiometrischen Menge von überschüssigem Kohlenstoff, um Niobcarbid zu bilden, so dass die eingestellte Zusammensetzung 1,0 bis 6,0 Gew.-% Niob und mindestens 3,3 Gew.-% Kohlenstoff enthält.

(iii) Herstellung einer geschmolzenen Charge aus der eingestellten Zusammensetzung bei einer Temperatur oberhalb des eutektischen Verfestigungspunkts;

(iv) Erniedrigung der Temperatur der geschmolzenen Charge zur Ausfällung des Niobcarbids oberhalb des eutektischen Verfestigungspunkts; und

(v) weitere Abkühlung der geschmolzenen Charge zur Bildung einer Menge von Graphit, die 2 bis 7% des Gesamtkohlenstoffs in der geschmolzenen Charge ist.

10. Verfahren nach Anspruch 9, dadurch gekennzeichnet, dass das Erniedrigen der Temperatur der geschmolzenen Charge das Erniedrigen der Temperatur mit nicht mehr als 1°C/s, bis im Wesentlichen der gesamte Carbid ausfällt, umfasst.

Revendications

1. Composition d'alliage convenant pour un cylindre refroidisseur à coquille ou une enveloppe de cylindre à coquille, la dite composition d'alliage consistant en:

1,0 à 6,0 % en poids de niobium ;

au moins 3,3 % en poids de carbone ;

et optionnellement

4,2 à 4,6 % en poids de nickel ;

0,3 à 0,5 % en poids de molybdène ;

1,5 à 2,0 % en poids de chrome ;

0,7 à 1,2 % en poids de silicium ;

0,7 à 1,0 % en poids de manganèse ; et

du fer et des impuretés ;

dans laquelle 2 à 7 % du dit carbone dans la dite composition d'alliage sont présents sous la forme de graphite libre.

2. Composition d'alliage selon la revendication 1, comprenant :

3,3 à 4,0 % en poids de carbone ;

1,0 à 6,0 % en poids de niobium ;

4,2 à 4,6 % en poids de nickel ;

0,3 à 0,5 % en poids de molybdène ;

1,5 à 2,0 % en poids de chrome ;

0,7 à 1,2 % en poids de silicium ;

0,7 à 1,0 % en poids de manganèse ; et

du fer et des impuretés.

3. Composition d'alliage selon les revendications 1 ou 2, comprenant de 1,0 à 3,0 % en poids de niobium.

4. Composition d'alliage selon une quelconque des revendications 1 à 3, comprenant environ 1,5 % en poids de niobium.

5. Composition d'alliage selon une quelconque des revendications 1 à 4, comprenant de 3,3 à 4,0 % en poids de carbone.

6. Composition d'alliage selon une quelconque des revendications 1 à 5, comprenant de 3,3 à 3,45 % en poids de carbone.

7. Procédé de fabrication d'un cylindre refroidisseur à coquille en fonte alliée selon une quelconque des revendications 1 à 6, le procédé comprenant :

la préparation d'une composition d'alliage de fer en fusion ;

l'ajustement de la dite composition d'alliage de fer par addition d'au moins un du niobium et d'un composé contenant du niobium pour obtenir une cuvée à l'état fondu contenant de 1,0 à 6,0 % en poids de niobium, et la fourniture d'une quantité stoechiométrique de carbone en excès dans la dite cuvée de fusion pour former du carbure de niobium et du graphite libre lors du refroidissement, la dite cuvée à l'état fondu incluant au moins 3,3 % en poids de carbone ; et

la coulée de la dite cuvée à l'état fondu pour former le dit cylindre à coquille contenant du carbure de niobium précipité et de sorte que 2 à 7 % du dit carbone sont précipités sous la forme de graphite libre.

8. Procédé selon la revendication 7, dans lequel la coulée de la dite cuvée à l'état fondu comprend le refroidissement de la dite cuvée à l'état fondu, à une vitesse non supérieure à 1°C/s environ, jusqu'à ce que sensiblement la totalité du carbure de niobium précipite.

9. Procédé de formation d'une composition d'alliage de fer comme défini dans une quelconque des revendications 1 à 6 convenant pour un cylindre ou un cylindre à coquille de laminoir, le procédé comprenant :

(i) la préparation d'une composition de cylindre à coquille indéfinié ayant un point de solidification eutectique auquel une teneur en graphite désirée se forme ;

(ii) l'ajustement de la dite composition par addition (a) d'au moins un du niobium et d'un composé contenant du niobium, et (b) d'une quantité stoechiométrique de carbone en excès pour former du carbure de niobium, de sorte que la composition ajustée comprend de 1,0 à 6,0 % en poids de niobium et au moins 3,3 % en poids de carbone ;

(iii) la production d'une cuvée à l'état fondu de la dite composition ajustée, à une température supérieure au point de solidification eutectique ;

(iv) l'abaissement de la température de la dite cuvée à l'état fondu, pour précipiter le dit carbure de niobium au-dessus du point de solidification eutectique ; et

(v) la poursuite du refroidissement de la dite cuvée à l'état fondu, pour former une quantité de graphite qui est de 2 à 7% du carbone total dans la dite cuvée à l'état fondu.

10. Procédé selon la revendication 9, dans lequel l'abaissement de la température de la dite cuvée à l'état fondu comprend l'abaissement de la température à une vitesse non supérieure à 1°C/s jusqu'à ce que sensiblement la totalité du dit carbure précipite.